Metformin HCl is hygroscopic, presents stability problems, and is not inherently compressible. Consequently, there is a need to provide a free-flowing and cohesive metformin HCl composition capable of being directly compressed into strong tablets with an acceptable in vitro dissolution profile.
Tablets may be defined as solid dosage pharmaceutical forms containing drug substances with or without suitable fillers. They are produced by compression or compaction of a formulation containing the drug and certain excipients selected to aid in the processing and to improve the properties of the product. Tablets may be coated or uncoated and are made from powdered, crystalline materials. They may include various diluents, binders, disintegrants, lubricants, glidants and in many cases, colorants. Excipients used are classified according to the function they perform. For example, a glidant may be used to improve the flow of powder blend in the hopper and into the tablet die.
There has been widespread use of tablets since the latter part of the 19.sup.th century and the majority of pharmaceutical dosage forms are marketed as tablets. Major reasons of tablet popularity as a dosage form among pharmaceutical manufacturers are simplicity, low cost, and the speed of production. Other reasons include stability of drug product, convenience in packaging, shipping, and dispensing. To the patient or consumer, tablets offer convenience of administration, ease of accurate dosage, compactness, portability, blandness of taste, ease of administration, and elegant distinctive appearance.
Tablets may be plain, film or sugar coated, bisected, embossed, layered, or sustained release. They can be made in a variety of sizes, shapes and colors. Tablets may be swallowed, chewed, or dissolved in the buccal cavity or beneath the tongue. They may be dissolved in water for local or topical application. Sterile tablets are normally used for parenteral solutions and for implantation beneath the skin.
In addition to the active or therapeutic ingredients, tablets may contain a number of inert materials known as excipients. They may be classified according to the role they play in the final tablet. The primary composition includes a filler, binder, lubricant, and glidant. Other excipients which give physical characteristics to the finished tablet are coloring agents, and flavors in the case of chewable tablets. Without excipients most drugs and pharmaceutical ingredients cannot be directly compressed into tablets. This is primarily due to the poor flow and cohesive properties of most drugs. Typically, excipients are added to a formulation to impart good flow and compression characteristics to the material being compressed. Such properties are imparted to these excipients through pretreatment steps such as wet granulation, slugging, spray drying spheronization, or crystallization.
Lubricants are typically added to prevent the tableting materials from sticking to punches, minimize friction during tablet compression, and allow for removal of the compressed tablet from the die. Such lubricants are commonly included in the final tablet mix in amounts usually less than 1% by weight.
In addition, tablets often contain diluents which are added to increase the bulk weight of the blend resulting in a practical size for compression. This is often necessary where the dose of the drug is relatively small.
Another commonly used class of excipients in tablets is binders. Binders are agents, which impart cohesive qualities to the powdered material. Commonly used binders include starch, and sugars such as sucrose, glucose, dextrose, and lactose.
Disintegrants are often included to ensure that the tablet has an acceptable rate of disintegration. Typical disintegrants include starch derivatives and salts of carboxymethylcellulose.
Other desirable characteristics of excipients include the following:
High compressibility to allow strong tablets to be made at low compression forces.
Good flow properties that can improve the flow of other excipients in the formula.
Cohesiveness (to prevent tablet from crumbling during processing, shipping and handling).
The three processes for making compressed tablets are wet granulation, direct compression, and dry granulation (slugging or roller compaction). The method of preparation and type of excipients are selected to give the tablet formulation the desired physical characteristics that allow for the rapid compression of the tablets. After compression, the tablets must have a number of additional attributes such as appearance, hardness, disintegrating ability, and an acceptable dissolution profile. Choice of fillers and other excipients will depend on the chemical and physical properties of the drug, behavior of the mixture during processing, and the properties of the final tablets. Preformulation studies are done to determine the chemical and physical compatibility of the active component with proposed excipients.
The properties of the drug, its dosage forms, and the economics of the operation will determine selection of the best process for tableting. Generally, both wet granulation and direct compression are used in developing a tablet.
The dry granulation method may be used where one of the constituents, either the drug or the diluent, has sufficient cohesive properties to be tableted. The method consists of blending, slugging the ingredients, dry screening, lubrication, and compression.
The wet granulation method is used to convert a powder mixture into granules having suitable flow and cohesive properties for tableting. The procedure consists of mixing the powders in a suitable blender followed by adding the granulating solution under shear to the mixed powders to obtain a granulation. The damp mass is then screened through a suitable screen and dried by tray drying or fluidized bed drying. Alternately, the wet mass may be dried and passed through a mill. The overall process includes: weighing, dry powder blending, wet granulating, drying, milling, blending lubrication and compression.
In general, powders do not have sufficient adhesive or cohesive properties to form hard, strong granules. A binder is usually required to bond the powder particles together due to the poor cohesive properties of most powders. Heat and moisture sensitive drugs cannot usually be manufactured using wet granulation. The large number of processing steps and processing time are problems due to high level manufacturing costs. Wet granulation has also been known to reduce the compressibility of some pharmaceutical excipients such as microcrystalline cellulose.
Direct compression is regarded as a relatively quick process where the powdered materials are compressed directly without changing the physical and chemical properties of the drug. The active ingredient(s), direct compression excipients and other auxiliary substances, such as a glidant and lubricant are blended in a twin shell blender or similar low shear apparatus before being compressed into tablets. This type of mixing was believed to be essential in order to prepare "pharmaceutically acceptable" dosage forms. For example, Remington's Pharmaceutical Sciences (RPS), pp 1203 to 1932 17.sup.th edition (1985), cautions pharmaceutical scientists that the manner in which a lubricant is added to a formulation must be carefully controlled. Accordingly, lubricants are usually added to a granulation by gentle mixing. RPS warns that prolonged blending of a lubricant with a granulation can materially affect hardness and disintegration time for the resulting tablets. Furthermore, Ansel et al (1995) Pharmaceutical Dosage Forms and Drug Delivery Systems, 6.sup.th Ed. p. 199, indicates that excessive blending of lubricants with the granulate ingredients cause water proofing of the granule and reduces tablet hardness or strength of the compressed tablet. For these reasons, high shear mixing conditions have not been used to prepare direct compression dosage forms.
The advantages of direct compression include uniformity of blend, few manufacturing steps involved, (i.e. the overall process involves weighing of powders, blending and compression, hence less cost), elimination of heat and moisture, prime particle dissociation, and physical stability.
Pharmaceutical manufacturers would prefer to use direct compression techniques over wet or dry granulation methods because of quick processing time and cost advantages. However, direct compression is usually limited to those situations where the drug or active ingredient has a crystalline structure and physical characteristics required to form pharmaceutically acceptable tablets. However, one or more excipients must often be combined with the active ingredient before the direct compression method can be used since many ingredients do not have the necessary properties. Since each excipient added to the formulation increases the tablet size of the final product, manufacturers are often limited to using the direct compression method in formulations containing a low dose of the active ingredient per compressed tablet.
A solid dosage form containing a high dose drug (i.e. the drug itself comprises a substantial portion of the total compressed tablet weight) could only be directly compressed if the drug itself has sufficient physical characteristics (e.g. cohesiveness) for the ingredients to be directly compressed.
For an example, metformin HCl (an oral hypoglycemic) is considered a high dose drug. Most tablet formulations include a range of 70 to 85% by weight of metformin HCl per tablet. This high dose drug, combined with its rather poor physical characteristics for direct compression, has not allowed pharmaceutical manufacturers to use direct compression as a method to prepare the final tablet.
For example, in U.S. Pat. No. 5,733,578 on acetaminophen to Hunter et al, acetaminophen could not be directly compressed with microcrystalline cellulose to form acceptable tablets. The final product tended to be soft, prone to capping and otherwise not pharmaceutically acceptable (i.e. difficult to swallow because of the large size). Consequently, the more time consuming and expensive wet granulation technique was used.
U.S. Pat. No. 4,661,521 on direct tableting acetaminophen composition to Salpekar et al, N-acetyl-p-aminophenol could only be directly compressed using additional steps such as slugging or roller compaction of the tableting mix. Salpekar et al employed fluidized bed apparatus for thorough blending of N-acetyl-p-aminophenol with pregelatinized starch. High shear mixing is used to form a slurry which is dried and fluidized again before sizing to achieve the appropriate particle size.
Another limitation of direct compression as a method of tablet manufacturing is the potential size of the compressed tablets. If the amount of active ingredient is high, a pharmaceutical formulator may choose to wet granulate the active ingredient with other excipients to attain an acceptable sized tablet with the desired amount of acetaminophen. The amount of filler, binder or other excipients needed in wet granulation is less than that required for direct compression since the process of wet granulation contributes toward the desired physical properties of the tablet.
Hydroxypropyl methylcellulose has been utilized in the pharmaceutical industry as a direct compression excipient for solid dose forms. Hydroxypropyl methylcellulose is a processed cellulose and controls drug release from solid dosage forms.
Despite the advantages of the direct compression, such as reduced processing time and cost, wet granulation is widely used in the industry to prepare solid dosage forms. Wet granulation is often preferred over direct compression because wet granulation has a greater chance of overcoming any problems associated with the physical characteristics of various ingredients in the formulation. This provides material which has the required flow and cohesive properties necessary to obtain an acceptable solid dosage form.
The popularity of wet granulation compared to direct compression is based on at least three advantages. First, wet granulation provides the material to be compressed with better wetting properties, particularly in the case of hydrophobic drug substances. The addition of hydrophilic excipients makes the surface of the hydrophobic drug more hydrophilic, reducing disintegration and dissolution problems. Second, the content uniformity of the solid dosage form is generally improved with wet granulation because all of the granules usually contain the same amount of drug. Lastly, the segregation of drug(s) from excipients is avoided.
Segregation could be a potential problem with direct compression. The size and shape of particles comprising the granulate to be compressed are optimized through the wet granulation process. This is because when a dry solid is wet granulated the binder "glues" particles together, so that they agglomerate into spherical granules.
In spite of the advantages afforded by wet granulation, many manufacturers would welcome the opportunity to directly compress tablets containing high dose metformin HCl. There is a need in the industry for techniques and pharmaceutical excipients which will allow manufacturers to prepare high dose metformin HCl tablets by direct compression which will avoid the time and expense of wet granulation.